There is known an actuator for valve for rotating the body of a valve by a motor to control the opening and closing of the valve and for adjusting the opening degree of the valve body to control the flow amount of a fluid passing through the valve. Such conventional valve actuator requires a number of reduction gears for reducing the rotary speed of the motor to a suitable rotary speed of the valve body, making the entire apparatus large-sized. Also, the conventional valve actuator requires an expensive motor incorporating a brake mechanism for preventing the valve body from being rotated due to the pressure of a fluid passing through the valve when the motor as drive means is stopped rotating. This presents the disadvantage of increase in manufacturing costs.